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Adsorption of acetylene on Si(100) is studied from first principles. We findthat, among a number of possible adsorption configurations, the lowest-energystructure is a ``bridge'' configuration, where the C$_2$H$_2$ molecule isbonded to two Si atoms. Instead, ``pedestal'' configurations, recently proposedas the lowest-energy structures, are found to be much higher in energy and,therefore, can represent only metastable adsorption sites. We have calculatedthe surface formation energies for two different saturation coverages, namely0.5 and 1 monolayer, both observed in experiments. We find that although, ingeneral, the full monolayer coverage is favored, a narrow range of temperaturesexists in which the 0.5 monolayer coverage is the most stable one, where theacetylene molecules are adsorbed in a $2\times 2$ structure. This resultdisagrees with the conclusions of a recent study and represents a possibleexplanation of apparently controversial experimental findings. The crucial roleplayed by the use of a gradient-corrected density functional is discussed.Finally, we study thermal decomposition of acetylene adsorbed on Si(100) bymeans of finite-temperature Molecular Dynamics, and we observe an unexpectedbehavior of dehydrogenated acetylene molecules.Comment: 8 pages, 3 figures (submitted to J. Chem. Phy

Acetylene on Si(100) from first principles: adsorption geometries, equilibrium coverages, and thermal decomposition